The Semipalmated Sandpiper (Calidris pusilla) is a small, abundant shorebird that breeds primarily in sub-Arctic to mid-Arctic habitats across the Nearctic and winters principally along the northern and central coasts of South America. No subspecies have been described and little is known concerning their genetics. However, birds show a cline in bill length across the Arctic, with longest bills in the east and shortest in the west. There appear to be several ‘steps’ in the cline, suggesting a division into eastern, central and western breeding populations. Since females average longer bills than males in a breeding population, there is considerable overlap of bill lengths at migration staging areas. Based on bill length patterns and sightings and recoveries of marked individuals, most western breeders migrate south through the prairies, along with some birds from central Arctic populations. The remaining central Arctic breeders, and all eastern Arctic birds, migrate south through the north Atlantic Coast of North America, particularly the Bay of Fundy. Western Arctic breeders appear to winter farther west in South America than eastern breeders, although there is considerable mixing among populations in French Guiana and Guyana. In spring, birds from the eastern Arctic migrate north through the U.S. Atlantic coast, including Delaware Bay. Central and western Arctic breeders primarily migrate north through the interior of North America. Therefore, central Arctic breeders in particular demonstrate an elliptical migration pattern.
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